U.S. Pat. No. 4,801,625 of W. J. Parr et al. describes the carbonate derivative of pentaerythritol phosphate as a good intumescent flame retardant for such polymers as polypropylene. It also describes that it can be manufactured by reacting pentaerythritol phosphate with carbonyldiimidazole. Since carbonyldiimidazole is quite expensive, its use makes the synthesis of the carbonate derivative rather expensive. A need exists for a less expensive synthetic procedure for the manufacture of the pentaerythritol phosphate carbonate flame retardant additive.